Foams containing urethane groups which are obtained by the reaction of polyisocyanates with polyether polyols and/or polyester polyols are known and widely used. The possibilities of using polyurethane foams are, however, severely restricted in certain fields due to their complete or relative inability to be high frequency welded or flame laminated. Areas in which these properties are particularly important include, for example, the manufacture of door facings in cars which requires the welding of a sheet of foam to another or to other materials, the production of quilted effects and the manufacture of shaped articles. Flame laminating is also used to produce composite systems containing textiles, e.g. as upholstery material which is subsequently shaped or molded by high frequency welding.
It is known that foams containing groups which are capable of being high frequency welded can be produced by reacting compounds which have several active hydrogen atoms with polyisocyanates in the presence of water and/or blowing agents, emulsifiers, stabilizers, catalysts and other auxiliary agents and compounds which render them capable of high frequency welding and flame laminating, or to subject the finished foams to an after-treatment which renders them capable of high frequency welding.
Compounds which confer such a high frequency welding quality include, for example, powdered polyvinyl acetates, polyvinyl chlorides and copolymers (Belgian Pat. No. 719,875), thermoplastic polymers such as ethylene vinyl acetate, cellulose derivatives such as ethyl or benzyl cellulose, acrylate polymers and polyethylene (German Offenlegungsschrift No. 1,769,583).
The ability of polyurethane foams to be high frequency welded can also be increased by the addition of water-soluble inorganic or organic salts such as sodium thiosulfate or sodium acetate (German Offenlegungsschrift 1,669,888). An improvement in the high frequency welding characteristics of polyurethane foams by after-treatment has been described, for example, in French Pat. Nos. 1,329,849; 1,344,444 and 1,343,681.
The addition of powders of thermoplast solutions to the starting materials used for the production of polyurethanes has certain disadvantages. To produce a significant effect, it is necessary to use substantial quantities, but this entails technical difficulties, for example due to the resulting sharp increase in viscosity, and leads to production difficulties. Moreover, the properties of the foams are adversely affected by these products. Subsequent impregnation of the finished foams is also not without its problems. It is very difficult, for example, to impregnate very thick layers of foam. Moreover, removal of the solvent and drying of the after-treated foam generally entails some technical difficulties and, in any case, after-treatment requires an additional operating step.
The known methods for the production of polyurethane foams which can be high frequency welded entail considerable technical (and financial) cost and result in foams which have unsatisfactory properties. Furthermore, polyether foams which have been treated by these methods are not left in a state in which they can be flame laminated, but require the addition of compounds such as derivatives of phosphorous or phosphoric acid at the stage of foaming. These compounds generally have a deleterious effect on the mechanical properties of the product.
It has now surprisingly been found that the additives according to the present invention substantially improve the capacity of polyurethane foams to be high frequency welded without having any adverse effect on their properties such as compression resistance and pressure deformation residue.